Spark plug for internal combustion engine

Abstract

A spark plug in which a front end portion of a ground electrode is positioned outside of a virtual outer circumferential face that is formed by extending a front and outer circumferential face of a center electrode in the axis direction, and positioned on a front end side in the axis direction with respect to a virtual face including a front end face of the center electrode. Further, the equation 1.1≦b/a≦1.6 is satisfied, where “a” (mm) represents a first minimal distance between the front end portion of the center electrode and the front end portion of the ground electrode and, where “b” (mm) represents a second minimal distance between the front end portion of a ceramic insulator and the front end portion of the ground electrode.

The invention claimed is: 
     
       1. A spark plug for an internal-combustion engine, comprising:
 a rod-like center electrode extending in an axis direction; 
 an insulator having an axial bore in the axis direction in which the center electrode is inserted; 
 a generally cylindrical metal shell provided on an outer circumference of the insulator; 
 a ground electrode extending from a front end portion of the metal shell and disposed so that a front end thereof is bent toward the center electrode; 
 a noble metal portion provided on the front end of the ground electrode which faces a front edge of the center electrode; and 
 a gap formed between the ground electrode and the center electrode, 
 wherein an entire front end portion of the ground electrode including the bent portion thereof is positioned outside of a virtual outer circumferential face that is formed by extending a front end outer circumferential face of the center electrode in the axis direction, and a terminating face of the ground electrode is positioned on a front end side in the axis direction with respect to a virtual face formed by extending a front end face of the center electrode, and 
 wherein the following equation is satisfied:
   1.1 ≦b/a≦ 1.6, 
 
 where “a” ( mm ) represents a first minimal distance between the front end portion of the center electrode and a point on the noble metal portion closest to the center electrode, and 
 where “b” ( mm ) represents a second minimal distance between the front end portion of the insulator and the same point on the noble metal chip. 
 
     
     
       2. The spark plug for an internal-combustion engine according to  claim 1 , satisfying the following equation:
   1.5 ≦b/a≦ 1.6 
 
       where “a” and “b” are as defined in  claim 1 . 
     
     
       3. The spark plug for an internal-combustion engine according to  claim 1 ,
 wherein, when a front end opening of the axial bore and a corner positioned closest to the front end portion of the center electrode are projected on a virtual projection face perpendicular to the axis, 
 an outer circumference length L of a projected axial bore between a first contact point and a second contact point on the ground electrode side occupies 40% or more of the outer circumference length of the projected axial bore, 
 where the first contact point is defined by a first tangent drawn from a first edge that is positioned on an end of a projected corner serving as the corner projected on the virtual projection face to the projected axial bore serving as the front end opening of the axial bore projected on the virtual projection face, 
 where the second contact point is defined by a second tangent drawn from a second edge that is positioned in the other end of the projected corner to the projected axial bore. 
 
     
     
       4. The spark plug for an internal-combustion engine according to  claim 3 ,
 wherein the length L between the first and second contact points along the outer circumference of the projected axial bore on the ground electrode side occupies 50% or more of the outer circumference length of the projected axial bore. 
 
     
     
       5. The spark plug for an internal-combustion engine according to  claim 1 , which includes a tapered portion in a front end portion of the axial bore that tapers off toward the front end in the axis direction. 
     
     
       6. The spark plug for an internal-combustion engine according to  claim 1 ,
 wherein a chamfered portion is formed in a front end opening of the axial bore. 
 
     
     
       7. The spark plug for an internal-combustion according to  claim 1 ,
 wherein a plurality of ground electrodes is provided. 
 
     
     
       8. The spark plug for an internal-combustion according to  claim 1 ,
 wherein the center electrode has a noble metal portion on at least a part of a portion facing the front end opening of the axial bore.